Dec 6, 2023 12:48 AM
An easy-sounding problem yields numbers too big for our universe
https://www.quantamagazine.org/an-easy-s...-20231204/
Researchers prove that navigating certain systems of vectors is among the most complex computational problems.
Might there be no quantum gravity after all?
https://physics.aps.org/articles/v16/203#c1
INTRO: Physicists’ best theory of matter is quantum mechanics, which describes the discrete (quantized) behavior of microscopic particles via wave equations. Their best theory of gravity is general relativity, which describes the continuous (classical) motion of massive bodies via space-time curvature.
These two highly successful theories appear fundamentally at odds over the nature of space-time: quantum wave equations are defined on a fixed space-time, but general relativity says that space-time is dynamic—curving in response to the distribution of matter. Most attempts to solve this tension have focused on quantizing gravity, with the two leading proposals being string theory and loop quantum gravity.
But new theoretical work by Jonathan Oppenheim at University College London proposes an alternative: leave gravity as a classical theory and couple it to quantum theory through a probabilistic mechanism. Such a hybrid strategy was traditionally considered a nonstarter, as it was thought to lead to inconsistencies.
Oppenheim avoids these pitfalls, but at the cost of having to insert probability—a “roll of dice”—into the evolution of space-time. Future experiments could test the viability of this approach by probing whether gravity is quantum.
For the past 70 years one of the most important problems in fundamental physics has been to reconcile quantum physics with general relativity. There are two strategies for this unification: either quantize gravity or find a way to insert quantum matter into a classical gravitational framework.
The former is clearly favored, but none of the quantum-gravity proposals have yet been experimentally confirmed. That would seem to leave an opening for the other strategy, but theorists have shown—through so-called no-go theorems—that coupling quantum matter to classical gravity leads to inconsistencies, such as violations of the famed Heisenberg uncertainty principle. Indeed, the best-known model for this quantum-to-classical coupling, the semiclassical Einstein equation, suffers from the inconsistencies predicted by these no-go theorems.
With his new approach, Oppenheim avoids the barriers of the no-go theorems by abandoning one of their underlying assumptions: that the coupling between classical gravity and quantum matter is reversible. In a reversible theory, the state of the system at any given time can be used, together with the equations of motion, to uniquely determine the state of the system at any other time in the past or the future.
However, not all theories need be reversible, they can also be stochastic. In a stochastic theory, the initial state of a physical system evolves according to an equation, but one can only know probabilistically which states might occur in the future—there is no unique state that one can predict... (MORE - details)
https://www.quantamagazine.org/an-easy-s...-20231204/
Researchers prove that navigating certain systems of vectors is among the most complex computational problems.
Might there be no quantum gravity after all?
https://physics.aps.org/articles/v16/203#c1
INTRO: Physicists’ best theory of matter is quantum mechanics, which describes the discrete (quantized) behavior of microscopic particles via wave equations. Their best theory of gravity is general relativity, which describes the continuous (classical) motion of massive bodies via space-time curvature.
These two highly successful theories appear fundamentally at odds over the nature of space-time: quantum wave equations are defined on a fixed space-time, but general relativity says that space-time is dynamic—curving in response to the distribution of matter. Most attempts to solve this tension have focused on quantizing gravity, with the two leading proposals being string theory and loop quantum gravity.
But new theoretical work by Jonathan Oppenheim at University College London proposes an alternative: leave gravity as a classical theory and couple it to quantum theory through a probabilistic mechanism. Such a hybrid strategy was traditionally considered a nonstarter, as it was thought to lead to inconsistencies.
Oppenheim avoids these pitfalls, but at the cost of having to insert probability—a “roll of dice”—into the evolution of space-time. Future experiments could test the viability of this approach by probing whether gravity is quantum.
For the past 70 years one of the most important problems in fundamental physics has been to reconcile quantum physics with general relativity. There are two strategies for this unification: either quantize gravity or find a way to insert quantum matter into a classical gravitational framework.
The former is clearly favored, but none of the quantum-gravity proposals have yet been experimentally confirmed. That would seem to leave an opening for the other strategy, but theorists have shown—through so-called no-go theorems—that coupling quantum matter to classical gravity leads to inconsistencies, such as violations of the famed Heisenberg uncertainty principle. Indeed, the best-known model for this quantum-to-classical coupling, the semiclassical Einstein equation, suffers from the inconsistencies predicted by these no-go theorems.
With his new approach, Oppenheim avoids the barriers of the no-go theorems by abandoning one of their underlying assumptions: that the coupling between classical gravity and quantum matter is reversible. In a reversible theory, the state of the system at any given time can be used, together with the equations of motion, to uniquely determine the state of the system at any other time in the past or the future.
However, not all theories need be reversible, they can also be stochastic. In a stochastic theory, the initial state of a physical system evolves according to an equation, but one can only know probabilistically which states might occur in the future—there is no unique state that one can predict... (MORE - details)